This invention relates in general to vehicular brake systems. In particular, this invention relates to a vehicle stability control (VSC) system having an evacuation valve for evacuating and filling an otherwise isolated circuit. This VSC system is adapted for used in an anti-lock brake (ABS) and traction control (TC) brake system.
Vehicles are commonly slowed and stopped with hydraulic brake systems. While these systems vary in complexity, a typical base brake system includes a tandem master cylinder, fluid conduit arranged in two similar but separate brake circuits, and wheel brakes in each circuit. The master cylinder generates hydraulic forces in both brake circuits by pressurizing brake fluid when the driver steps on the brake pedal. The pressurized fluid travels through the fluid conduit in both circuits to actuate brake cylinders at the wheels and slow the vehicle.
Braking a vehicle in a controlled manner under adverse conditions requires precise application of the brakes by the driver. Under these conditions, a driver can easily apply excessive brake pressure thus causing one or more wheels to lock, resulting in excessive slippage between the wheel and road surface. Such wheel lock-up conditions can lead to greater stopping distances and possible loss of directional control.
Advances in braking technology have led to the introduction of ABS systems. An ABS system monitors wheel rotational behavior and selectively applies and relieves brake pressure in the corresponding wheel brakes in order to maintain the wheel speed within a selected slip range while achieving maximum braking forces. While such systems are typically adapted to control the braking of each braked wheel of the vehicle, some systems have been developed for controlling the braking of only a portion of the braked wheels.
Electronically controlled ABS valves, comprising apply (isolation) valves and dump valves, are located between the master cylinder and the wheel brakes and perform the pressure regulation. Typically, when activated, these ABS valves operate in three pressure control modes: pressure apply, pressure dump and pressure hold. The apply valves allow brake pressure into the wheel brakes to increase pressure during the apply mode, and the dump valves release pressure from the wheel cylinders during the dump mode. Wheel cylinder pressure is held constant during the hold mode.
A further development in braking technology has led to the introduction of traction control (TC) systems. Additional valves have been added to existing ABS systems to provide a brake system that controls wheel speed during acceleration. Excessive wheel speed during vehicle acceleration leads to wheel slippage and a loss of traction. An electronic control system senses this condition and automatically applies braking pressure to the wheel cylinders of the slipping wheel to reduce the slippage and increase the traction available. In order to achieve optimal vehicle acceleration, braking pressures greater than the master cylinder pressure must quickly be available when the vehicle is accelerating.
During vehicle motion such as cornering, dynamic forces are generated which can reduce vehicle stability. A VSC brake system improves the stability of the vehicle by counteracting these forces through selective brake actuation. These forces and other vehicle parameters are detected by sensors that signal an electronic control unit. The electronic control unit automatically operates pressure control devices to regulate the amount of hydraulic pressure applied to specific individual wheel brakes. In order to achieve optimum vehicle stability, brake pressures greater than the master cylinder pressure may be required in a very short time. However, a brake system that generates high pressures very quickly typically has high power requirements or uses a large high pressure accumulator.
During installation of a brake system, an evacuation and fill process removes air trapped in the system and fills the system with hydraulic brake fluid. In order to reduce installation time, it is desirable to perform an evacuation and fill process without opening otherwise normally closed valves, particularly solenoid actuated valves which would require electrical connection. In a VSC system, it is desirable to also evacuate and fill isolated circuits without electrically connecting priming and charging valves.
This invention relates to an electronically controlled vehicular brake system providing vehicle stability control. The vehicular brake system includes a hydraulic master cylinder connected to wheel brakes via brake conduits. A pump generates fluid pressures and pressure control valves located between the master cylinder and the wheel brakes regulate the fluid pressures at the wheel brakes to achieve ABS and traction control. A medium pressure accumulator stores fluid pressurized by the pump that is supplied to the wheel brakes via associated control valves to achieve VSC braking control. An evacuation valve is provided in the conduit in which the medium pressure accumulator is provided. The evacuation valve permits evacuation and fill of an otherwise isolated conduit without increasing cycle times or complexity of procedures during installation of the system. Additionally, the evacuation valve can be used during service of brake system if desired.